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Thibaut Montmerle, Jean-Philippe Lafore, and Jean-Luc Redelsperger

Abstract

Results from a three-dimensional cloud model are extensively compared with airborne Doppler radar data in the case of a tropical oceanic squall line observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The comparison is based on the precipitation patterns, the dynamical and thermodynamical distributions, and the vertical transport of horizontal momentum.

The model simulates the evolution of the mesoscale convective system (MCS) frontal convective line from a quasi-linear to a broken pattern. The area located south of the “break,” which designates the region where the MCS leading edge reorientates from the N–S to the E–W direction, is composed of a pronounced bow-shaped structure with two vortices located on both sides of a strong rear inflow.

The vertical circulation is characterized by a jump updraft and an overturning downdraft. Both structures exhibit a vertical, intense updraft in the break zone, whereas the jump updraft is more sloped and less intense in the bow region. Front-to-rear momentum is injected mainly by the jump updraft. Both observations and simulation indicate the major role played by convective eddies in the vertical transport of cross-line and parallel-line horizontal momentum.

A synthesis summarizes the complex three-dimensional structure of the simulated system, based on three salient features and their relative locations: the deep convection region, the leading edge of the cold pool, and the melting area. The relative positions between the two last mentioned explains the observed asymmetric structure and the existence of more upright and narrow updrafts in the northern part of the system. Numerical experiments suggest that the wind profile at midlevel is mainly responsible for the location of the melting area relative to the cold pool. The system tends to generate new convective elements organized along the direction that reduces the angle between the convective line and the midlevel shear vector.

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